1. Field of the Invention
This invention relates to the conversion of sulfur-laden hydrocarbon fuels to product gas containing hydrogen, carbon monoxide, and trace amounts of hydrocarbons that is suitable for use in solid oxide fuel cells. In one aspect, this invention relates to a fuel processor system for converting sulfur-laden hydrocarbon fuels to fuels suitable for use in fuel cells. More particularly, this invention relates to a fuel processor system for converting heavier, sulfur-laden hydrocarbon fuels, such as JP-8 and diesel fuels, to fuels suitable for use in high temperature fuel cells, such as solid oxide fuel cells.
2. Description of Related Art
Fuel cells are electrochemical devices that convert the chemical energy of a fuel into electrical energy with high efficiency. The basic physical structure of a fuel cell consists of an electrolyte layer with a porous anode electrode and porous cathode electrode on opposed sides of the electrolyte. In a typical fuel cell, gaseous fuels, typically hydrogen frequently obtained by the reforming or gasification of higher hydrocarbon fuels, are continuously fed to the anode electrode and an oxidant, typically oxygen from air, is continuously fed to the cathode electrode. The electrochemical reactions take place at the electrodes to produce an electric current.
In a solid oxide fuel cell, the electrolyte is a solid, nonporous metal oxide, normally Y2O3-stabilized ZrO2 (YSZ), the anode electrode is a metal/YSZ cermet and the cathode electrode is typically Sr-doped LaMnO3. The solid oxide fuel cell operating temperature is typically in the range of about 650° C. to about 1000° C., at which temperature ionic conduction by oxygen ions occurs. Due to the limited amount of power which individual fuel cells are able to produce, individual fuel cells are typically assembled to produce a fuel cell stack comprising a plurality of fuel cells to enable the production of higher power outputs.
Most fuel cells are adversely affected by the presence of sulfur compounds in the fuel gas. While pure hydrogen is the ideal fuel gas for all fuel cell types, its availability is extremely limited. One solution is to convert more readily available and easily transportable hydrocarbon fuels such as natural gas, liquid petroleum gas, alcohols, gasoline and diesel fuel to hydrogen by means of a reforming process. Several reforming technologies to produce hydrogen are known, including autothermal reforming, partial oxidation reforming, plasma reforming, and steam reforming. Reforming of natural gas or other hydrocarbons produces hydrogen-enriched products which, in addition to hydrogen, may also include carbon monoxide, carbon dioxide, and carbon. At the present time, about 90% of the hydrogen produced around the world is from reforming natural gas, as a result of which demand for natural gas is increasing considerably.
Recently, efforts to develop various kinds of fuel reformers to reform liquid or gaseous fuels to produce hydrogen-enriched fuels have increased substantially. Most of these reformers use steam reforming technology, which requires heat and steam. Steam reforming involves the endothermic reaction of hydrocarbon or alcohol fuel with steam to produce carbon monoxide and hydrogen. Steam reformers use high temperature catalyst filled tubes heated by burners fueled by any number of means including fuel cell exhaust fuel and air streams. Steam may be supplied by a waste heat boiler. Heat transferred through the tube walls drives the endothermic reaction.
Generally, heavier liquid hydrocarbons such as diesel fuels and JP-8 are the most difficult to reform due to possible incomplete reforming and the tendency of the reforming process to produce soot rather than the desired product gas. Most of these fuels tend to comprise significant amounts of sulfur which, if allowed to remain in the reformed fuel, may have detrimental effects on the performance of fuel cells. In addition, heavier hydrocarbons which may be present may also have detrimental effects on the performance of the fuel cells.
Several methods are known to those skilled in the art for removing sulfur, and the method employed depends on both the reforming system employed and the type of fuel. If catalytic reforming is employed, sulfur is typically removed from the feedstock prior to reforming. Hydrodesulfurization, in which hydrogen from the product gas stream is reacted with the fuel over a catalyst to convert the sulfur compounds to hydrogen sulfide after which the hydrogen sulfide is removed by passing the stream through a zinc oxide (ZnO) bed, is one well known sulfur removal process for use in connection with liquid hydrocarbons. U.S. Pat. No. 5,686,196 to Singh et al. teaches a system for operation of a solid oxide fuel cell generator on diesel fuel which includes a hydrodesulfurizer which reduces the sulfur content of commercial and military grade diesel fuel in which hydrogen, which has previously been separated from the process stream, is mixed with the diesel fuel at low pressure after which the resulting diesel fuel/hydrogen mixture is pressurized and introduced into the hydrodesulfurizer. The hydrodesulfurizer comprises a metal oxide such as ZnO which reacts with hydrogen sulfide in the presence of a metal catalyst to form a metal sulfide and water. After desulfurization, the diesel fuel is reformed and transferred to a hydrogen separator which removes most of the hydrogen from the reformed fuel prior to introduction into a solid oxide fuel cell generator. The separated hydrogen is then selectively transferred to the diesel fuel/hydrogen mixer or to a hydrogen storage unit.
For desulfurizing natural gas before reforming, activated charcoal filtration is usually all that is required. Non-catalytic partial oxidation reformers are able to tolerate sulfur in the fuel and convert it to hydrogen sulfide which can be removed from the product gas by passing the product gas through a zinc oxide bed. For high temperature fuel cells (temperatures in the range of about 600° C. to about 1000° C.), the reforming process may be carried out at the nickel anode surface using the steam, carbon dioxide, and heat from the power generation reaction.
As an alternative to fuel reforming for producing hydrogen for use in a fuel cell, U.S. Pat. No. 6,653,005 B1 to Muradov teaches a compact hydrogen generator coupled to or integrated with a fuel cell for portable power applications in which hydrogen is produced by thermocatalytic decomposition (cracking, pyrolysis) of hydrocarbon fuels in an oxidant-free environment. The apparatus is indicated to be suitable for use with a variety of hydrocarbon fuels including sulfurous fuels such as natural gas, propane, gasoline, kerosene, diesel fuel, and crude oil. The catalysts for hydrogen production in the apparatus are carbon- or metal-based materials and doped, if necessary, with a sulfur-removing agent.
A significant issue in the operation of high temperature fuel cells is heat management, in particular minimizing the amount of heat loss. Conventional fuel cell power systems for operation of high temperature fuel cell stacks are limited in thermal integration for heat recovery because of the use of discrete heat exchangers, which require extensive ducting and thermal insulation. This approach has made these fuel cell systems both complex and costly to manufacture and tends to place constraints on fuel cell stack design configurations to support the required plumbing system. To address this issue, U.S. Pat. No. 5,612,149 to Hartvigsen et al. teaches a fuel cell module with a fuel cell column having at least one fuel cell stack, mated with the planar wall of a heat exchanger, wherein the fuel cell column and heat exchanger are mounted to a support structure, and which define an air plenum between the fuel cell column and the planar wall of the heat exchanger, thereby eliminating the ductwork and insulation requirements associated with heat exchange systems while increasing the efficiency of the heat exchanger. However, the disclosed design only provides for single stage heating of the oxidant inlet by a single heat exchanger which would not raise the ambient air for the oxidant to the required operating temperature range of the solid oxide fuel cell stack due to the very limited surface and residence time to which the gas would be subjected. In addition, other key requirements such as fuel feedstock preheating prior to reformation, heating needs during system start-up from ambient conditions and partial load operations are also not addressed by this disclosure.
U.S. Pat. No. 4,943,494 to Riley teaches porous refractory ceramic blocks arranged in a stack configuration providing both support and coupling means for a plurality of solid oxide fuel cells. The ceramic blocks and the outer steel shell of the structure provide connections for the air, fuel and process effluent flows. One of the main objects of the disclosed structure is to provide a support structure that integrates fuel, air and effluent flow channels for reduction of interconnection complexities for cost reduction and commercial feasibility. However, the disclosed structure does not provide any means for heat recovery, which is critical for efficient operation and cost effective system operation.
U.S. Pat. No. 5,763,114 to Khandkar et al. teaches a thermally integrated reformer located inside of a furnace structure housing solid oxide fuel cell stacks. In this system, heat from the fuel cell oxidation reaction is recovered to support the endothermic reformation reaction. Heat is recovered by heat transfer to the reformer by radiation from the fuel cell stack and by forced convection from the exhausting airflow exiting the furnace. Although addressing the need for heat recovery and transfer to the fuel feedstock as well as support for the reformation reaction, the heating of the air for the oxidant feedstock is not integrated and is provided by external means resulting in system inefficiency and fabrication complexity. An apparatus for heat recovery is also taught by U.S. Pat. No. 5,906,898 to Pondo, which teaches a fuel cell stack with oxidant flow paths between separator plates and along the outside surface of the fuel cell stack for control of the heat generated by the fuel cells. This patent also teaches direct heating of the oxidant feed gas by using recovered heat from the fuel cell stack by way of heat exchange panels mounted externally on the fuel cell stack, providing oxidant inlet flow paths to the fuel cell stack. However, the highest temperature effluent stream is not fully utilized in this configuration for heat recovery because of its containment inside of the fuel cell stack in the oxidant outlet internal manifold.
U.S. Pat. No. 7,169,495 B2 to Pastula et al. teaches a thermally integrated fuel cell system comprising a fuel cell stack zone which includes one or more fuel cell stacks, a secondary reformer, a radiative heat exchanger, and an equalization heat exchanger, a burner zone which includes an afterburner, a primary reformer, and a high temperature heat exchanger, and a low temperature zone which includes a low temperature heat exchanger and a steam generator. In operation, the fuel is combined with steam and passed sequentially through the primary reformer and the secondary reformer. Air is split into two parallel streams and preheated in the low temperature heat exchanger. One air stream passes through to the high temperature exchanger while the other passes to the radiative heat exchanger. The air and fuel streams are equalized in the equalization heat exchanger before entering the fuel cell stack. The stack exhaust is combusted in the afterburner, the exhaust from which heats the primary reformer, the high temperature heat exchanger, the low temperature heat exchanger, and the steam generator.
Yet another approach to thermal management of a fuel cell and fuel cell system is taught by PCT International Publication No. WO 03/098728 A1. Disclosed therein is a method for thermal management in which a fuel supply stream comprising hydrogen, steam, at least one carbon oxide and optionally methane is processed using a methanator to produce a fuel supply stream comprising a controlled concentration of methane and the fuel cell methane present in the fuel cell supply stream is reformed. The methanator is operated in a manner in which adjustments are made in response to fluctuations in the temperature of the fuel cell such that the concentration of methane in the fuel cell supply stream is controlled to achieve a desired level of reforming of methane within the fuel cell.